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Since ancient times about 2500 open clusters have been found in our galaxy, but astronomers estimate that the true number could total 30,000. As one might guess, the majority of these are invisible because they are located in regions of the Galaxy hidden from our direct view, but a significant number of these, hidden by dust in the Galactic plane, should be accessible to our infrared telescopes.
The dust associated with regions of star formation absorbs and scatters the photons emitted by young stars. At optical wavelengths this absorption can dim the clusters by factors of 10 thousand to 100 million, rendering them all but invisible. However, at infrared wavelengths the effects of absorption are much less severe, and although the infrared light is still attenuated, many clusters remain bright enough to be detected.
This is one of the reasons that ever larger telescopes have been built that are dedicated to studies at infrared wavelengths, at it is the largest of these, ESO's 4.1 metre VISTA telescope, that has discovered 96 new open clusters in a single study. Some of these new clusters, hidden until now within the dust of the Milky Way, are shown above.
The multiple discovery was made as part of the VISTA Variables in the Via Lactea programme (VVV) one of the 6 principal surveys being carried out by various research teams with VISTA (which stands for, Visible and Infrared Survey Telescope for Astronomy).
The 96 new clusters are the most spectacular result so far of the VVV programme, which started last year and is due to last for a total of 5 years and make use of almost 2000 observing hours.
It is still early days then, but the results so far reached (soon to be published in Astronomy & Astrophysics) show that most of the clusters are small, with only 10-20 stars. Whether this tells us something important about how stars form in very dusty regions, or is the result of how the survey was carried out, remains to be seen.
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